Semiconductor nanocrystals have a wide variety of applications. Of the many unique properties of these materials, the photophysical characteristics may be the most useful. Specifically, these materials can absorb light and then emit an intense luminescent emission that is particle size-dependent and particle composition-dependent. This fluorescent emission can have an extremely narrow luminescence bandwidth, can be environmentally sensitive or insensitive depending on the nanocrystal's structure, and can be resistant to photobleaching under intensive light sources. Emissions can be efficiently excited with electromagnetic radiation having a shorter wavelength than the highest energy emitter in the material, and by varying the size and composition of the nanocrystal, a user can use many different types of nanoparticles mixed together and can still distinguish each type. These properties allow semiconductor nanocrystals to be used as markers or as ultra-sensitive luminescent reporters of biological states and processes in highly multiplexed systems.
Nanocrystals are typically spherical or nearly so (though methods of making nanocrystals of other shapes are known), and can have multiple layers, such as a central core, a surrounding shell, and optional capping groups, linkers, and other surface-conjugated materials. Typically, core/shell nanocrystals are described according to the composition of the core and of a semiconductor shell applied outside the core; the shell usually stabilizes the nanocrystal and protects its photophysical properties. It may also provide an attachment surface for linking the nanocrystal to a molecule, cell, subcellular organelle, and the like that is to be tracked or observed.
The nanocrystal core largely determines its critical light absorption and emission characteristics. Nanocrystal cores have been broadly studied and improvements in synthesis have led to the optimization of key physiochemical properties resulting in nanocrystal cores with uniform size distributions and intense, narrow emission bands following photo-excitation. However, nanocrystal cores alone lack sufficiently intense or stable emission intensities for most applications, and nanocrystal cores are particularly sensitive to their environment; for example, the aqueous environment required for many biological applications can lead to the complete destruction of the luminescence of nanocrystal cores. Thus, methods to photostabilize nanocrystal cores (e.g., protect their luminescent properties) and make them stable and useful in aqueous media are of great interest for biological applications. Commonly, this is achieved by applying a shell over the core, to form a so-called core/shell nanocrystal.
The choice of shell material must be made to match the core material. For example, the shell material may have a wider band gap than the core, which enables it to protect the activated state that the core occupies when it has been photoactivated, forming a separated electron and hole. The shell may ideally be chosen to have an atomic spacing and lattice structure that closely match those of the core material to best preserve the photophysical attributes of the core, since irregularities in the interface between core and shell may be responsible for non-radiative energy dissipation mechanisms that reduce luminescent efficiency.
Core/shell nanocrystals having a CdX core wherein X is S, Se, or Te coated with a YZ shell where Y is Cd or Zn, and Z is S, Se, or Te are commonly discussed and used, and have been shown to have good emission characteristics and stability. This may largely be due to the YZ coating material's band-gap energy which spans that of the core relatively symmetrically. ‘Symmetry’ as used in this sense means that the wider bandgap of the shell material fully encompasses the narrower bandgap of the core material and extends both above the high end of the core material's bandgap and below the low end of the core material's bandgap.
One limitation of CdSe-based core/shell nanocrystals is that the blue emitting particles have lower extinction coefficients than red emitting particles. This is due to the fact that emission wavelength are tuned by changing the CdSe core particle size: smaller particles have a blue-shifted emission, but also typically absorb light less efficiently than larger, red-shifted particles. Several researchers have shown that by utilizing alloy cores (e.g., CdSSe or ZnCdSe) one can tune the wavelength by adjusting the elemental composition rather than size, and can thus decouple emission color from extinction coefficient. One can also utilize a semiconductor material with a larger bulk band gap such that the largest nanocrystals emit in the blue/green portion of the visible spectrum (e.g., ZnSe).
A more serious limitation of CdSe nanocrystals for certain applications such as in vivo imaging or diagnostic tests is toxicity. Cadium is a toxic metal. The toxicity of cadmium, and to a lesser extent selenium, raises concerns about using a nanocrystal containing cadmium and selenium for in vivo applications in live organisms or in living cells. Therefore, bright and stable nanocrystals that do not contain cadmium are of special value for such uses, and for any uses involving large scale production or use of nanocrystals, in order to minimize environmental impact and associated health concerns.
Accordingly, for certain applications it is advantageous to use different core materials that do not have attendant toxicity concerns. Additionally, for some applications, very small or very large nanocrystals (relatively speaking) may be advantageous; for example, if used to label a biomolecule like DNA or a protein, it may be preferable to have a very small nanocrystal, less than about 10 nm in overall size, including the core/shell nanocrystal and a coating used on the shell to adapt the particle for use in a suitable medium. For biomolecules, the most relevant medium is frequently water; thus the nanocrystals must often be specially treated and/or coated so they are readily suspended or dissolved in water. For other applications, such as tracking a large cell such as a bacterium, flow cytometry, cellular imaging, protein blotting, and other protein detection methods, it may be advantageous to use a single, very bright nanoparticle, which may sometimes be a larger particle.